1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine and a printer, and more particularly, to an image forming apparatus in which transfer of toner from a toner carrier toward a back electrode is controlled so that the toner adheres to an image receiving member, such as a transfer paper, a copying paper, a recording paper and a transfer medium, and an image is accordingly formed.
2. Description of the Related Art
Most widely used as an image forming method using toner are the jumping development and the projection development. According to these types of image forming methods, an electrostatic latent image corresponding to an image is formed on a photosensitive member and toner serving as an image forming agent is made adhering to the electrostatic latent image on the photosensitive member from a toner carrier, such as a development roller, so that a toner image is created, and after this, the toner image is transferred onto a transfer paper, a copying paper, a recording paper or the like. However, since these image forming methods require a step of forming an electrostatic latent image on a surface of a photosensitive member (exposing process), there is a problem that an apparatus has a complicated structure and a cost regarding the apparatus and a running cost are expensive. Noting this, over the recent years, as disclosed in U.S. Pat. No. 3,689,935, U.S. Pat. No. 5,036,341 and International Publication No. WO 98/24634, an increasing attention has been paid to an image forming apparatus of the so-called xe2x80x9cTonerJet (Registered trademark)xe2x80x9d method which requires to transfer toner onto an image receiving member, such as a transfer paper, a recording paper and an intermediate transfer belt, under the function of an electric field to thereby form an image.
In such an image forming apparatus, disposed as toner transfer controlling means between a back electrode and a toner carrier which carries and transports electrified toner is a flexible printed circuit (hereinafter referred to as xe2x80x9cFPCxe2x80x9d) which comprises a plurality of toner passing apertures which pass electrified toner, and an image receiving member such as a recording paper and a transfer medium is disposed between the FPC and the back electrode. As a potential difference is applied between the toner carrier and the back electrode, an electrostatic field for transfer of toner develops which makes electrified toner transfer from the toner carrier toward the back electrode. Further, in the FPC, a control electrode is disposed around each toner passing aperture, and as a voltage applied upon each control electrode in accordance with an image signal is controlled, each toner passing aperture is electrostatically opened and closed, the electrified toner is made transfer from the toner carrier toward the back electrode through the toner passing apertures in accordance with the image signal mentioned above, and the toner adheres to the image receiving member. In this manner, a toner image corresponding to the image signal is formed on the image receiving member.
In the case of such an image forming apparatus, for stable creation of a toner image, it is necessary to maintain a small and constant gap between a toner carrier and a control electrode. Noting this, as disclosed in Japanese Patent No. 3045295, Japanese Patent Application Laid-Open Gazette No. 2000-211180 and the like, a gap keeping technology has been proposed which requires to dispose a thin spacer (gap defining means) between a toner carrier and an FPC to thereby keep the gap constant. In other words, as the spacer is inserted between the toner carrier and the FPC, the gap is maintained constant to approximately the thickness of the spacer.
Meanwhile, for the purpose of forming a toner image whose density is less uneven and whose image quality is excellent, a toner layer to be formed on a toner carrier must be uniform. However, in an image forming apparatus as above wherein a spacer is disposed between a toner carrier and an FPC, a toner layer on the toner carrier is in contact with the spacer. Hence, in conventional apparatuses, the uniformity of the toner layer on the toner carrier is disturbed as the toner layer contacts the spacer, and the image quality accordingly degrades in some cases.
In order to stably form a toner image having an excellent image quality, therefore, it is necessary to meet two requirements, one for a constant gap and the other for the uniformity of a toner layer on a toner carrier. However, there has been no apparatus which satisfies the both requirements.
A major object of the present invention is to provide an image forming apparatus in which the uniformity of a gap and the uniformity of a toner layer both improve and a toner image having an excellent image quality is formed stably.
The present invention is directed to an image forming apparatus which comprises: a back electrode; a toner carrier which transports a toner layer, which is formed by electrified toner, to a toner transfer starting position which is faced with the back electrode while carrying the toner layer and moving in a predetermined travel direction; and toner transfer controlling means which is disposed between the toner carrier and the back electrode and controls toner transfer from the toner transfer starting position on the toner carrier toward the back electrode so that toner arrives at an image receiving member which is transported between the back electrode and the toner transfer controlling means and a toner image is accordingly formed.
In fulfillment of the foregoing object, a first aspect of the image forming apparatus further comprises gap defining means which comes in contact with the toner layer and accordingly defines a gap between the toner carrier and the toner transfer controlling means. The gap defining means has a width in a width direction, which is perpendicular to the travel direction of the toner carrier, smaller than a width of the toner layer in the width direction and comes in contact with the toner layer within both edge portions of the toner layer in the width direction.
According to a second aspect of the present invention, the image forming apparatus further comprises gap defining means which defines a gap between the toner carrier and the toner transfer controlling means. The gap defining means is made of a material having a work function approximately the same as a work function of the toner, and comes in contact with the toner layer at an upstream position to the toner transfer starting position in the travel direction of the toner carrier.
According to a third aspect of the present invention, the image forming apparatus further comprises toner restricting means which restricts quantity of the toner layer on the toner carrier to a predetermined value at an upstream position to the toner transfer starting position in the travel direction of the toner carrier; and gap defining means which comes in contact with the toner layer at a position between the toner transfer starting position and the toner restricting means and defines a gap between the toner carrier and the toner transfer controlling means. The image forming apparatus is characterized in that the toner restricting means restricts a thickness of the toner layer to smaller than 2d where the symbol d denotes a volume mean diameter of the toner.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.